1. Field of the Invention
The present invention relates to an injection molding machine screw driving gear comprising a rotary motor and a hydraulic chamber that is especially suitable for use with a screw preplasticization type injection molding machine, and more particularly to the support and lubricating structure of a piston member for axially moving the screw.
2. Description of the Prior Art
Recently, the applicant proposed a screw pre-plasticization injection molding machine as shown in laid-open Japanese patent No. 3-97518 that is provided with a rotary motor (electric or hydraulic) to turn a kneading screw and a hydraulic actuator to move the screw axially. In plasticizing and weighing, the screw retreats to form an opening between the screw tip and a path for sending melt resin to an injection chamber of an injection part, while in injecting, the path is closed, with the tip of the screw maintained at the forward position. This makes it possible for the machine to prevent melt resin from flowing backward to the plasticizing chamber during injection without using a check valve.
The screw driving gear of the injection molding machine is connected, as shown in FIG. 4, with a link shaft 2 via a spline 4 and a position adjustment device 8 at the back of a screw 1. The link shaft 2 is also connected with an output shaft 5a of a rotary motor 5 by way of a spline 3. In addition, the link shaft 2 is rotatably supported by a piston member 7 via bearings 6,6. The piston member 7 forms a hydraulic chamber 10 together with a casing 9.
Thus, rotation of the rotary motor 5 is transmitted to the screw 1 by way of the link shaft 2 at the time of plasticizing so that the screw melts and kneads the resin before it presses out the melted resin to the injection chamber. A reaction force from the melt resin then acts on the screw 1 in a direction indicated by an arrow A. The reaction force is received by the draining of hydraulic chamber 10 of the casing 9 by way of the position adjustment device 8, link shaft 2, bearings 6,6 and the piston member 7.
The rotary motor 5 pauses during the injection process, and pressurized oil is supplied to the hydraulic chamber 10 through an oil path 10a to axially move the piston member 7, forwarding the screw 1 in a direction indicated by an arrow B by way of the bearings 6,6 and link shaft 2. The tip of the advanced screw closes the path to the injection chamber. The screw 1 needs a high pressurizing power to cope with the injection pressure, and the pressurizing power acts on the bearings 6,6.
Accordingly, the screw driving gear of the injection molding machine extends radially, because the piston member 7 is provided on the outer circumference of the link shaft 2 by way of the bearings 6,6. The bearings 6,6 need to support the rotation of the screw 1 while resisting the reaction force therefrom at the time of plasticizing. On the other hand, they must be angular conic roller bearings or the equivalent to be capable of supporting a big thrust force while keeping a high pressurizing power in the injection process. Since the bearings 6, 6 are arranged inside the piston member, a moveable member, it is difficult to supply them with lubricating oil. This will make them less durable and prevent them from being downsized, because they must be of a large capacity for safety reasons.